1.1. What Is a Model?

A model is a way to organize a system’s functions and features to define its structural design. A design can help us understand how a communication system accomplishes tasks to form a protocol suite. To help wrap our heads around models, communication systems are often compared to the postal system (Figure 1-1). Imagine writing a letter and taking it to the post office. At some point, the mail is sorted and then delivered via some transport system to another post office. From there, it is sorted and given to a mail carrier for delivery to the destination. The letter is handled at several points along the way. Each part of the system is trying to accomplish the same thing—deliver the mail. But each section has a particular set of rules to obey. While in transit, the truck follows the rules of the road as the letter is delivered to the next point for processing. In between, inspectors and sorters ensure the mail is metered and safe, without much concern for traffic lights or turn signals.